Rank filters are currently implemented in image and video processing systems. A rank filter is an image-processing algorithm that replaces each pixel p by its rank relative to the pixels in some neighborhood of p. Thus if p is the brightest pixel in its neighborhood, it is mapped to white. If p is the darkest pixel, it is mapped to black. Further, if p is the median pixel, it is mapped to 50% (gray).
The problem with current rank filter algorithms is that a relatively large magnitude of processing time is required to generate an output. For example, up to 30 comparison steps may occur, demanding up to ten clock cycles of processing time.